In the related art, wrinkles are generated when forming an image on a thick sheet material using a fixing device. In order to suppress generation of the wrinkles, a method of producing a low pressure by changing a position of a biasing member biasing a pressing roller with a cam and weakening a biasing force is known. However, in this method, it is difficult to stabilize the biasing member at a plurality of different positions. As a result, it is difficult to realize optimal pressure depending on the sheet materials having various thicknesses.